1. Field of the Invention
This invention relates to an improved catalyst composition. More specifically, it pertains to an improved platinum-zeolite type catalyst composition having a long life, excellent heat resistance and high activity and selectivity for conversion of aromatic hydrocarbons, for example the isomerization of xylenes and reforming of naphtha.
2. Description of the Prior Art
Zeolite, or crystalline aluminosilicate, generally has high activity in the conversion reactions of hydrocarbons, such as cracking, hydrocracking, reforming, isomerization and alkylation. Many catalysts compositions based on zeolite have therefore been proposed to date. Isomerization of xylenes is one typical reaction which is carried out in the presence of a zeolite-base catalyst.
Isomerization of xylenes is industrially performed by the steps, in suitable combinations, of isomerizing an aromatic hydrocarbon stock containing mainly xylene isomers, separating a specified xylene isomer, normally p-xylene, from the resulting isomerization reaction mixture, and recycling the mixture left after the separation. It is industrially significant in this case, for an increased efficiency of the isomerization reaction and a reduced cost of production, to adjust the composition of the xylene isomers in the isomerization reaction product as closely as possible to the thermodynamic equilibrium composition, and to inhibit side-reactions such as the decomposition of xylenes (particularly, the hydrogenation of the benzene ring) and disproportionation reaction.
Many methods for isomerizing xylenes have been suggested in the past, and many of them involve the use of a crystalline aluminosilicate zeolite-containing catalyst. Extensive work has been done to improve and develop catalysts and improve the isomerization reaction conditions in regard to the aforesaid prior methods, and a number of suggestions have been made as a result of such work. In particular, much research efforts have been concentrated on methods involving changing the shape or structure of the zeolite catalyst itself; methods involving modifying the zeolite catalyst by subjecting it to a physical treatment, for example heat-treatment, and methods involving chemically modifying the zeolite catalyst by adding various ingredients. For example, there have been suggested a method in which Y-type zeolite is treated with super heated steam to improve its activity and stability (see U.S. Pat. No. 3,887,630), and a method in which MoO.sub.3 is supported on offretite to improve its activity to decompose ethylbenzene (see U.S. Pat. No. 3,848,009).
None of the prior suggested catalysts for isomerization of xylenes completely meets two contradictory requirements (a) and (b) below. (a) To have superior activity on the isomerization of xylenes, and (b) to greatly reduce undesirable side-reactions (such as the hydrogenation of the benzene ring, hydrogenolysis, demethylation, and particularly disproportionation and transalkylation).
In the case of isomerizing xylene isomers containing ethylbenzene, it is desirable to deethylate ethylbenzene in addition to the isomerization reaction of the xylenes, and some methods for this purpose have been suggested, for example as seen in U.S. Pat. Nos. 4,098,836, 4,163,028, and 4,152,363.
In the previously suggested methods, however, undesirable side-reactions such as hydrogenation of the benzene ring, disproportionation of xylenes and transalkylation of xylenes and ethylbenzene take place in addition to the isomerization of xylenes and the deethylation of ethylbenzene, and a loss of xylenes cannot be avoided.
For example, the aforesaid three U.S. Patents disclose a method for isomerizing xylene isomers containing ethylbenzene using ZSM-series zeolites modified with a metal of Group VIII of the Periodic Table such as platinum or nickel. With ZSM-series zeolite catalysts modified with nickel (with a nickel content of at least 2% by weight), demethylation of xylene is promoted under severe reaction conditions in which the conversion of ethylbenzene is high, and the loss of xylene increases. It has therefore been considered to be advantageous in industrial operation to use a platinum-group metal which induces little demethylation. ZSM-series zeolites modified with platinum have superior activity of isomerizing xylenes and superior ability to deethylate ethylbenzene selectively. However, platinum itself has a high ability to hydrogenate the benzene ring, and the hydrogenation occurs markedly as the temperature decreases owing to thermodynamic equilibrium. Consequently the amount of naphthenes formed increases and a loss of xylene increases. Accordingly, ZSM-type zeolite catalysts modified with platinum need to be used in industrial applications at temperatures of as high as more than 800.degree. F. (427.degree. C.). The temperature required for the isomerization reaction is affected by the space velocity, but generally, temperatures of about 300.degree. to 340.degree. C. are sufficient. At high temperatures, the isomerization is not improved, but rather undesirable side-reactions such as disproportionation and transalkylation are promoted to cause an increased loss of xylenes.
In order to avoid such undesirable reactions as much as possible, a method has also been suggested in which the space velocity based on the zeolite catalyst is increased to increase the optimum isomerization temperature to a level higher than in ordinary methods and therefore to promote deethylation while inhibiting a loss of xylenes attributed to disproportionation, etc. (see U.S. Pat. No. 4,152,363). With this method, however, it is difficult to maintain the isomerization at a high level, and the degradation of the catalyst increases because the temperature and the space velocity are high.